In offshore petroleum production systems, the petroleum produced by wells located on the ocean floor is carried to stationary production units (SPU) using pipes. The set of pipes used to inject water and pump oil and gas is conventionally known as flow lines.
This set of pipes, which comprises the flow lines, can be basically split into two different types: rigid and flexible, and extend from the producing well to the platform, to which they are connected using support structures especially designed to support and resist the stresses that result both from their own weight and their movement.
Flexible flow lines may be described as being ducts comprising the overlaying at least six independent layers of completely different composition: the first and innermost layer is the inner carcass, followed by a polymeric pressure sheath; this is followed by the pressure armour, the internal tensile armour and the external tensile armour, which comprise braided steel wire. Over all of these, we have an external polymeric sheath.
A preliminary inventory of the company's huge industrial park shows that there are currently some 5,000 kilometers of flexible ducts operating in the different Brazilian sedimentary basins, with diameters ranging from 7 to 45 centimeters.
Thus, the collection of flexible ducts in Brazil is quite heterogeneous. Different production units have flexible ducts of different diameters and different ages. Added to this scenario, we have the internal pressure caused by the fluids conveyed, the external pressure exerted by the environment, the internal friction between the various layers comprising the duct, corrosion and fatigue. In addition to these, there are all manner of influences caused by the environment in which structure is assembled, such as marine currents, salinity, changing tides, waves, incrustations with living organisms and external impacts.
This data demonstrates the value of preventive maintenance techniques and the importance of searching for efficient and quick techniques for corrective maintenance.
In spite of all the precautions taken to prevent failure, among the failures that can happen is a characteristic phenomenon that is regularly found in flexible ducts, known as birdcaging. This structural failure is the result of, among other factors, sudden and instantaneous depressurising of the duct, which causes radial expansion of the outermost tensile armour through tears in the outer sheath.
Briefly, the failure is due to tears in the outer polymeric sheath, in general associated with the effects caused by said depressurisation of the duct, causing the steel braid lying immediately under it to be exposed to an aggressive medium. Furthermore, the absence of any sheath effect exerted by the outermost polymer layer on the section of duct bearing the tear allows the internal pressure of the duct to push the successive layers in this region outwards, further exposing the steel braid that was uncovered by the tear, thus giving rise to failures that look like a “birdcage” (BC).
This type of failure not only changes the standard distribution of the stresses along the various layers comprising the flexible duct, it also results in faster action of the aggressive medium on the outer structural steel braid and the polymer layer immediately under it, which are exposed directly to the marine environment.
Defects such as these seriously compromise the integrity of the line, weakening the wires in the duct structure due to corrosion. Thus, as a safety measure, it is recommended that use of the duct be suspended until the damaged section can be replaced with a new one.
According to current prior art, this is achieved by replacing the damaged section. However, an intervention to replace a section of flexible duct where birdcaging has been detected involves mobilizing special PLSV (Pipe-Laying Support Vessels) and drilling rigs, making the operation quite costly. The intervention also takes a long time, as much as 15 to 16 days.
One must also consider the availability of special vessels and equipment to perform the repair procedure. Production losses to prepare the line are considerable, and currently there are no alternative methods to correct this type of defect in flexible ducts.
It is clear that a large petroleum concern requires a technical and administrative structure dedicated to the periodic inspection and correction of sections that are damaged or under imminent risk of leakage. Despite all efforts to this effect, the extension and different types of flexible ducts that comprise the Brazilian industrial park results in a vendor funnel in terms of the availability of the materials required, such as flexible duct sections and connections of a diameter corresponding to the damaged sections. These hurdles can delay such an intervention for months.